The invention relates generally to ink and coating material metering systems. Most metering systems can be classified in one of five general categories.
FIG. 1 shows a two-roll ink metering system having an elastomeric fountain roll, an applicator roll, and an ink transfer roll. A plate roll cylinder and impression cylinder complete the printing or coating station. The latter two cylinders are not part of the ink metering system and can be changed to a single roll if a plate roll is not needed.
FIG. 2 shows a regular-angle doctor blade system where an ink transfer roll is immersed in a fountain of ink as the ink transfer roll carries up the ink. A doctor blade is positioned at an acute angle toward the unmetered ink to smooth the ink off the ink transfer roll.
FIG. 3 shows a reverse-angle doctor blade system where an ink transfer roll is immersed in a fountain of ink. A doctor blade is positioned at an obtuse angle toward the unmetered ink so that ink is sheared off the ink transfer roll as the ink transfer roll carries up the ink.
FIG. 4 shows a combination system where a fountain roll acts as an applicator roll and does a certain amount of initial metering. The final metering is usually accomplished by a reverse-angle doctor blade rather than a regular-angle blade.
FIG. 5 shows a dual doctor blade system where the ink is pumped into a cavity between two doctor blades. One doctor blade is a regular-angle doctor blade, and the other is a reverse-angle doctor blade. The system can be used in either rotation direction. The cavity is kept under slight pressure. The blade holder is attached to the press frame and can be moved to control blade pressure and cleanup.
The two-roll system has a fountain roll which is usually Buna N, a synthetic rubber, and an engraved ink transfer roll. The engraved roll contains a number of cavities per lineal inch. The fountain roll supplies ink to the cavities, which prevent ink squeeze-out. The desire to print the fine line screens of tone and process work and thinner coating thickness brought about the development of a new series of engraved rolls. Ceramic coated and laser engraved rolls are the latest development. Such rolls give long repeatable results, high cavities count and randomly placed cavities which eliminate the interference patterns referred to as moires, often developed when printing the fine line screen with a conventional pyramidal mechanically engraved roll.
A differential ink metering system was devised in an attempt to run at higher speeds without slinging ink. The fountain roll was slowed down in relation to the speed of the ink transfer roll for non-differential ink metering. In addition to curtailing the ink slinging problem, the system proved to have several other advantages. The definite wiping action improved the metering. Also, when using an engraved roll, the fountain roll wiping action forced ink into the cavities under pressure which helped to fill the cavities. The main drawback to this system is that the quantities of ink or coating delivered are a function of speed. As the speed of the fountain roll increases, the hydraulic pressure of the ink or coating increases, depressing the rubber fountain roll and allowing more ink to pass through the nip between the fountain roll and the ink transfer roll.
In the regular-angle doctor blade system, the metering action is similar to the differential systems in that the metal blade is deflected upward at greater speeds and, consequently, greater hydraulic ink pressures. In many systems, the angle of the blade to the engraved cylinder can be varied to obtain optimum metering conditions.
In the reverse-angle doctor blade system, the doctor blade is at an obtuse angle toward the ink supply. The blade supplies a shearing action which cleans excess ink from the ink transfer roll. Larger particles are deflected by the blade. The hydraulic pressures of the ink push the blade into the engraved roller. At higher speeds the ink transfer is just about the same as it is at slower speeds. The metal blade causes greater wear to the engraved transfer roll, especially at higher press speeds, due to hydraulic pressure of the ink pushing the blade into the engraved roll making metal-to-metal contact. This wear has been found to be less at some blade angles. Also, some different blade materials have been found preferable. Most systems are Swedish clock-spring steel, although several plastics have been used at varying degrees of success when rusting of metal blades is a problem. The plastics previously used have been nylon, epoxy, and polyester. In general, these blades wear much faster than the steel. Most plastic swells and eventually deteriorates in the presence of solvent and causes the blade not to stay in contact with the engraved roll.
In doctor blade systems where the transfer roll is immersed in ink, starvation can occur at higher speeds. The combination doctor blade/fountain roll system solves the starvation problem, but has not eliminated the hydraulic effect at high speed. It also adds cost to the system and increases power usage.
The dual blade system solved most of the problems associated with the other systems. It maintains proper ink volume at all press speeds, reduces solvent loss, prevents contamination of the ink, reduces ink starvation because the cavity is pressurized, and eliminates the use of the rubber fountain roll. Due to the complicated mechanism of the holder and its being attached to a press frame that is not always rigid, the blades have often experienced differing or excessive pressure resulting in rapid wear.
A deterioration of a doctor blade surface occurs because of the wiping action on the engraved cylinder. Generally, doctor blades exhibit two basic types of wear which may be classified as adhesive wear and abrasive wear. Adhesive wear is caused by the shearing action of micro-contacts formed between two surface asperities that actually carry the load between the blade and the cylinder. This type of wear occurs because of the break in continuity of the ink or coating film which should separate the two surfaces.
Abrasive or cutting type of wear takes place whenever hard, foreign particles are present between the rubbing surfaces. Depending upon severity, abrasive wear may be of a gouging or scratching form on the cylinder. However, abrasive wear has virtually been eliminated by the development of the ceramic coated and laser engraved metering cylinder.
Adhesive wear of the blades is controlled by the lubrication properties of the ink, cylinder surface finish and working pressure of the blade resulting from the adjustment force. When the lubrication film separating two surfaces is interrupted, blade to cylinder contact strongly accentuates the blade wear. This phenomenon is caused by insufficient fluid or high blade pressure which creates an insufficient bearing effect.
Accordingly, it an object of the present invention to solve the problems of wear, ink and coating slinging, and lack of uniform metering due to the drawbacks of conventional ink metering systems described above.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.